The present embodiments relate to medical use of ultrasound. In particular, acoustic pressure of ultrasound adapts for diagnostic or therapeutic needs.
Ultrasound contrast agents are stabilized microbubbles. Contrast agents are pressure sensitive. High acoustic pressures destroy the microbubbles. In order not to destroy microbubbles, the transmit power is typically reduced by 10 dB-30 dB compared to typical B-mode imaging conditions. Moreover, the non-linear response of a microbubble may be weaker than the linear response of a scatterer in tissue. Thus, the signal-to-noise ratio (SNR) of contrast images is low, so using the maximum power that does not break the microbubbles is desirable.
Adjusting the transmit power to improve SNR or to minimize bubble destruction interferes with the data acquisition as the adjustment can only be performed after microbubbles have entered the imaging plane. Before a higher-quality acquisition may be repeated, the microbubbles have cleared the blood stream (e.g., about 15 minutes). Moreover, multiple injections of contrast agent are undesirable from a patient safety perspective and from a cost perspective.
In an attempt to set the transmit power before injection of contrast agents, clinicians use the mechanical index (MI) value as an indicator of acoustic pressure. While the MI is related to acoustic pressure, MI does not consider the patient's anatomy and does not provide information about the distribution of acoustic pressure in the imaging plane. Experienced clinicians may make adjustments based on the SNR of the B-mode image, but this is guesswork.
For ultrasound therapy, a desired thermal dose is to be applied to the patient. To much thermal dose may harm healthy tissue, and too little thermal dose may not fully treat the patient. However, various intervening structures may result in unexpected thermal dose. Like contrast agent imaging, the ability to predict the actual pressure is limited.
Other therapeutic applications may require certain acoustic pressure levels, for example to release drugs from microbubbles by destroying the shell or to establish stable cavitation.